1. Field of the Invention
The present invention relates to a phosphor for use in a vacuum fluorescent display, and more specifically to a ZnO:Zn type phosphor which is used in a vacuum fluorescent display and which has an increased initial luminance and a decreased drop of luminance with time.
2. Description of Related Art
In the prior art, a ZnO:Zn type phosphor has the most excellent luminous efficiency among various phosphors used in a vacuum fluorescent display. This vacuum fluorescent display is one type of evacuated display tube in which the anodes are coated with a phosphor that glows when electrons from the cathode strike it, to create a display. In the current situation in which the vacuum fluorescent display is widely used in a video tape recorder and as a display means in automobiles, the ZnO:Zn phosphor has already become one indispensable material for the vacuum fluorescent display.
The ZnO:Zn phosphor used in the conventional vacuum fluorescent display is an n-type semiconductor of oxygen-deficient type, and therefore, has gas absorption property (absorbing H.sub.2 O, CO.sub.2, CO, O.sub.2, etc.) which is as high as it is used as a gas sensor material. Because of this property, the ZnO:Zn phosphor absorbs various gases in the process of a manufacturing the vacuum fluorescent display, and in particularly, in a heat treatment process of the manufacturing process (sintering of the ZnO:Zn phosphor, and sealing and evacuation of the vacuum fluorescent display), a surface of the ZnO:Zn phosphor is oxidized in comparison with a condition adjusted as the phosphor material. As a result, an initial luminance was low.
In operation, gas is emitted from constitution members of the vacuum fluorescent display, due to a heating caused by an electric power consumption, or a use environment temperature. The emitted gas is absorbed by a getter, but a portion of the emitted gas remains within the vacuum fluorescent display. On the other hand, as mentioned above, since the ZnO:Zn phosphor has an intense gas absorption property, the ZnO:Zn phosphor absorbs the remaining emitted gas, and as a result, the luminance decreases with time.
In order to improve the characteristics of the ZnO:Zn phosphor, various researches has been made and are being made at present. Japanese Patent Application Laid-open No. Sho 58-40746 proposed that tungsten oxide powder is mixed into the ZnO:Zn phosphor. This was effective in preventing the "decrease of luminance with time". However, the initial luminance rather dropped.